Why 'Mrs.' isn't read as 'mistress'?

It is read as mistress, albeit a contracted form of it, just like ma'am is a popular contraction of madam. Quoting WP:

Mrs. originated as a contraction of the honorific Mistress, the feminine of Mister, or Master, which was originally applied to both married and unmarried women. The split into Mrs. for married women from Ms. and Miss began during the 17th century.

It is rare for Mrs. to be written in a non-abbreviated form, and the word lacks a standard phonetic spelling. In literature it may appear as missus or missis in dialogue. A variant in the works of Thomas Hardy and others is "Mis'ess", reflecting its etymology.

Mr(s) is a reference to the feminine of "Mr." The most common reference is to "missus," (a female "mister" if you will), although it could also be a reference to "mistress" (a female master).